AAGAM AND NIGAM

Aagam:

28 Tantras as revealed by Lord Shiva are called Mool Aagam

Nigam:

Tantra as revealed by Devi Parvati,consort of Lord Shiva

Lord Shiva initiated 5 Rishis into Aagam first.

Kaushik
Kashyap
Bhardwaj
Atri
Gautam

Afterwards, they made different Sampradayas and became Shaiva,Shakth,Ganpatya etc.Later when human intelligence and memory started declining tantra books were written.